It is generally known to use radar systems for determining the distance or range from the radar system to moving or stationary target objects, and/or for determining the radial velocity or the relative velocity of such target objects. The radar systems are typically adapted for use in different distance ranges. The main field of use of such radar systems is typically in long range applications with a relatively large distance, either up to 150 km or up to 300 km, depending on the application, between the radar system and the target object. An example of such a long range application of radar systems is in the field of aviation, for purposes of air traffic control or for navigation of aircraft.
Recently, on the other hand, applications have also arisen for radar monitoring in very short distance ranges between the radar system and the target objects. For example, such a close range or short range is up to 20 m or 250 m depending on the particular application. One such a short range application of a radar system is in the field of motor vehicles, for monitoring the traffic space around a particular motor vehicle, e.g. for determining the separation distance or spacing of the subject motor vehicle from other preceding, following, or approaching motor vehicles or other reflection objects or target objects. The radar system may alternatively or additionally be used for determining the relative velocity of the subject motor vehicle with respect to other preceding, following, or approaching motor vehicles or other reflection objects.
In such a radar system, an oscillator generates a high frequency analog transmitted signal, having a frequency in the GHz range, typically between 18 GHz and 94 Hz. Then a transmitting antenna emits this transmitted signal into an observation area, i.e. an area that is to be observed for monitored by the radar. After the signal transits the transmission path and is reflected back from the reflection objects located in the observation area, the reflection or echo signal is detected as a received signal by a receiving antenna. Then, the received signal is evaluated in a signal processing circuit with regard to the transit time and/or the frequency shift and/or the phase shift of the reflected received signal relative to the original transmitted signal. From this evaluation, the necessary distance information and/or velocity information can be determined.
Two different types of radar systems are generally in use, namely a pulsed radar system and a frequency modulated continuous wave (FMCW) radar system. These two types of radar systems are distinguished from each other based on the measuring principle that is carried out, and especially in the manner of generating the transmitted signal and in the time sequence or progression of the transmitted signal itself.
In a pulsed radar system, the transmitted signal is cyclically interrupted, in other words, such a radar system emits transmitted pulses having a respective determined pulse duration. During the respective pulse pause interval between each two successive transmitted pulses, the reflection signals resulting from reflections or echoes of the preceding transmitted pulse are detected as received signals. As such, the pulsed radar system operates with alternating transmitting and receiving operating phases. The distance or range from the radar system to the reflection objects is determined by a direct measurement of the signal transit time. The desired distance resolution of the pulsed radar system can be prescribed and selected by a corresponding selection of the pulse duration or pulse width of the transmitted pulses. To select a distance range, the signal processing of the received signal typically uses a plurality of distance or range gates, which respectively correspond to various signal transit times and thereby are selective for a very particular distance.
It is very easy to achieve a decoupling of transmitted and received signals in the operation of the pulsed radar system. In other words, a side-to-side crosstalk of the transmitted signal into the received signal can be completely prevented by a suitable switching from the transmitting operation to the receiving operation, for example by means of transmit-receive switches. Moreover, the dynamic range of the received signal that is to be processed, i.e. the input dynamics in the detection of the received signal, can be significantly reduced by prescribing a range-dependent amplification to be carried out in the signal processing, by means of a regulation of the sensitivity of the signal amplification dependent on the transit time, commonly known as "Sensitive Time Control" (STC).
In an FMCW radar system on the other hand, the transmitted signal is continuously emitted as a continuous wave (CW), wherein the transmitting frequency of the transmitted signal is varied by frequency modulation (FM) to have a predetermined frequency modulation characteristic over time. The received signal is detected simultaneously while the transmitted signal is being continuously emitted. In view of the simultaneous transmitting and receiving operations, a rather high expense and effort is necessary for achieving an adequate decoupling of the transmitted signals from the received signals. This results in a high cost for such a radar system, and also results in interfering side effects.
In view of the difficulties or disadvantages encountered with FMCW radar systems, it is commonly the practice to use pulsed radar systems or particularly pulse Doppler radar systems in short range applications as mentioned above. For example, such pulsed radar systems are typically used for monitoring the traffic space surrounding a subject motor vehicle so as to detect reflection objects represented by obstacles and/or other motor vehicles within this traffic space, whereby the distance or range, the relative velocity, and the position of the reflection objects can be determined. An important characteristic parameter for judging the quality of the pulsed radar system is the angle measuring accuracy, i.e. the angular resolution of the azimuth angle by which different reflection objects can be discriminated or distinguished from each other.
In order to increase the resolution capability, it is known to use a plurality of antennas (i.e. at least two antennas) in a radar system. The publication "Radar Handbook" edited by M. Skolnik and published by McGraw-Hill, 1990 (2nd Edition), describes a so-called monopulse radar method, in which a transmitted signal is emitted in a broad transmitted beam, and two receiving antennas are arranged symmetrically relative to the transmitted beam for detecting the reflection signals (respectively in relation to the azimuth angle and the elevation angle). An evaluating circuit evaluates the difference and the sum of the two received signals provided by the two antennas respectively in relation to the azimuth angle and the elevation angle, and thereby the angular positions of the reflection objects can be determined.
Published European Patent Application 0,499,706 discloses a so-called pulse Doppler radar method and system in which the same antenna is used for emitting the transmitted signal and for detecting the received signal. By means of appropriate switching and frequency shifting, a single oscillator is used for generating the transmitted signal and for generating an internal or local oscillator signal used in connection with the reception.